Traditionally, electromechanical or single-function solid-state relays have been designed for use in electric power systems to meet the requirements of synchronous generator protection. Major advances in digital technology and signal-processing algorithms make it now possible to integrate many protective functions into a single digital relay, providing an economically viable alternative for the protection of the generator. In addition, digital technology provides improved performance and greater flexibility.
In earlier prior art digital protective relays, the microprocessor simply replaced discrete relay logic and the voltage and current signals are processed by many analog components. The performance of these prior art protective relays depends on the accuracy of analog components used and is subject to dc offsets which drift with temperature, supply voltage changes, or aging. In later digital protective relays, the microprocessor both processes the signals and performs the logic, thereby eliminating the problems associated with analog hardware and providing a simpler design and better performance.
A prior art multifunction protective relay incorporates many protective functions in a single relay where an analog signal-processing hardware is replaced with a digital signal processor (DSP), as disclosed in U.S. Pat. No. 5,224,011 issued to Murty V. V. S. Yalla, et al. In this patent, the voltage and current input signals of the relay are modeled as sinusoidal signals which may be affected by dc offset and harmonic components. These voltage and current input signals can be characterized by various parameters, namely, rms value, peak value and frequency of the fundamental frequency component. These various parameters of the input signals are estimated using digital signal-processing algorithms.
Relays for protection of synchronous generators connected to electric power systems take into account the types of faults and abnormal operating conditions that may be present at the generator and the connected power system. Many such faults can be detected by sensing the generator terminal voltages and currents.